1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac lead having a shape-memory polymeric breakaway structure, such as a passive fixation tine or the distal end of a lead sleeve.
2. Description of the Related Art
Conventional cardiac stimulator systems consist of a cardiac stimulator and an elongated flexible cardiac lead that is connected proximally to a header structure on the cardiac stimulator and is implanted distally at one or more sites within the heart requiring cardiac stimulation or sensing. The cardiac stimulator is normally a pacemaker, a cardioverter/defibrillator, a sensing instrument, or some combination of these devices.
At the time of implantation, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip or, alternatively, by a passive mechanism, such as one or more radially spaced tines that engage the endocardium. The proximal end of the lead is then connected to the cardiac stimulator and the incision is closed.
One of the body's normal physiological responses to the engagement of a lead with the endocardium is the production of fibrous tissue around the distal end of the lead. The production of fibrous tissue begins almost immediately after the lead is fixed to the heart and may continue as long as the lead is in place. The extent of fibrous tissue growth varies from patient to patient. In most patients, the tissue envelops several millimeters of the distal end of the lead. However, depending upon the patient's particular physiology, as well as the duration of implantation, the extent of fibrous tissue developed may be much more substantial.
For many endocardial leads, fibrous tissue serves the vital function of providing a relatively permanent anchor for the distal end of the lead. Indeed, screw-in tips and tines are designed to hold the lead in contact with the endocardium until fibrous tissue develops and takes over the job of anchoring the lead in position.
It is not uncommon for recipients of cardiac stimulator systems to require replacement of their existing cardiac leads. There are a variety of medical indications that may prompt lead replacement. Some examples include deterioration of the patient's arrhythmia condition due to disease or injury, new sites of electrical dysfunction within the heart, and lead failure. Where lead replacement is indicated, the physician may choose to remove the existing lead or leave the existing lead in place and implant and connect a new lead to the cardiac stimulator.
The initial step in a normal lead removal procedure involves gently tugging proximally on the proximal end of the lead in an attempt to dislodge the distal end of the lead from the endocardium. Since it is normally desirable to remove the lead intact, care must be taken during this operation to avoid applying excessive thrust to the lead that might otherwise break the lead into sections or disconnect the insulating sleeve of the lead from the electrode structures. If the lead is unintentionally broken during the removal process, the physician may have difficulty successfully removing all of the lead components via the original tissue pathway used to implant the lead. In such circumstances, it may be necessary to insert a femoral catheter and enter the heart through the inferior vena cava in an attempt to fish out the remaining components of the lead. The removal process may be hampered by the presence of the fibrous tissue gripping the lead.
In circumstances where tissue ingrowth is substantial enough to prevent ready removal of the lead, physicians sometimes resort to the introduction of a fibrous tissue cutting tool, which consists of a tubular introducer that is passed over the lead body to the site of tissue ingrowth. The tip of the introducer includes a cutting implement, such as a serrated blade or a laser fiber optic element that is used to cut through the fibrous tissue. Extreme care must be exercised when using this type of fibrous tissue cutting instrument to avoid perforating the walls of the heart. Even with careful administration, some fibrous tissue may be so substantial that cutting may not be successful.
Although a decision to leave an existing lead in place avoids some of the difficulties associated with lead removal, there are, nevertheless, trade-offs associated with such a course of treatment. In circumstances where an existing lead is left in place and a new lead is implanted and connected to the cardiac stimulator, the heart must accommodate an additional tubular structure. Where the existing lead is in the right ventricle, the result is another structure interacting with the patient's tricuspid valve. For some patients, the problem may be compound. There are examples of arrhythmia patients undergoing six such lead replacement procedures whose hearts contain six or more disconnected cardiac leads.
Infection represents another potential problem associated with leaving an existing lead in place following replacement with a new lead. A disconnected cardiac lead may present a pathway for infections originating outside of the heart to bypass the protective function of the pericardial sac and attack the heart from within. This problem seems to arise more frequently in geriatric patients who experience infection in their subcutaneous tissue pockets as a result of the gradual thinning of the dermal layers adjacent to their cardiac stimulators. An infection originating in the subcutaneous pocket of a patient may quickly travel through the lumen of a disconnected lead and attack the myocardium.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.